The goal of plant breeding is to combine various desirable traits in a single variety. Such desirable traits may include greater yield, resistance to diseases, insects or other pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, enhanced growth rate and improved fruit properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same genotype. A plant cross-pollinates if pollen comes to it from a flower of a different genotype.
Plants that have been self-pollinated and selected for (uniform) type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny of homozygous plants. A cross between two such homozygous plants of different lines produces a uniform population of hybrid plants that are heterozygous for many gene loci. The extent of heterozygosity in the hybrid is a function of the genetic distance between the parents. Conversely, a cross of two plants each heterozygous at a number of loci produces a segregating population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties requires the development of homozygous inbred plants, the crossing of these inbred plants to make hybrids, and the evaluation of the hybrids resulting from the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selfing and selection of desired phenotypes. The new plants are evaluated to determine which have commercial potential.
Tomato (Solanum lycopersicum and closely related species) is naturally a diploid and the basic chromosome number of the genus is x=12, most are 2n=2x=24, including the cultivated ones. It originated in the New World and has since become a major food crop.
Tomato cultivars may be grouped by maturity, i.e. the time required from planting the seed to the stage where fruit harvest can occur. Standard maturity classifications include ‘early’, ‘midseason’ or ‘late-maturing’. Another classification for tomatoes is the developmental timing of fruit set. ‘Determinate’ plants grow foliage, then transition into a reproductive phase of flower setting, pollination and fruit development. Consequently, determinant cultivars have a large proportion of the fruit ripen within a short time frame. Growers that harvest only once in a season favor determinant type cultivars. In contrast, ‘indeterminate’ types grow foliage, then enter a long phase where flower and fruit development proceed along with new foliar growth. Growers that harvest the same plants multiple times favor indeterminate type cultivars.
Tomatoes can also be classified by their target markets: fresh market and processing tomatoes. Fresh-market tomatoes are primarily used for salads, salad bar and sandwiches, and require good storage properties. On the other hand, processing tomatoes generally requires red colored and pink to red/crimson fruit flesh and higher percentage of soluble solids. Processing tomatoes can be canned whole, canned, diced or chopped, dried, roasted, pasted, puréed or concentrated, juiced, frozen, or put into ready-made dishes, for example, sauces, stews or soups.
In 2017, World Atlas reported that the worldwide production of tomatoes amounted to 170.8 million tons. United States is ranked as the third largest producer of tomatoes in the world, next to China and India. Tomatoes are available in the United States year-round, with California and Florida being the major producers. Fresh-market tomatoes are available from May to December although supply peaks in July and in September through October. Processing tomatoes have the greatest supply from August to September.
In response to more recent consumer demands for dietary diversity, tomato breeders have developed a wider range of colors. In addition to expanding the range of red colored fruits, there are cultivars that produce fruits that are creamy white, lime green, yellow, green, golden, orange and purple. Additionally, there are multi-colored varieties exemplified by mainly red fruited varieties with green shoulders, and both striped- and variegated-colored fruit.